]]>The eyes of the common moth have led to a novel discovery that could boost the efficiency of solar panels. Scientists at Brookhaven National Laboratory have developed a way to create an ultra-tiny texture on silicon (which is the most common material for solar panels) that cuts down on a solar panel’s reflection in the same way that a moth’s eyes operate.

A moth’s compound eyes have textured patterns made up of tiny posts, with each one smaller than the wavelength of light. When light hits the moth’s eye, much of it is absorbed and passes into its cornea without disruption. The moth’s eye evolved in this way so that it could see better at night, and so that its eyes wouldn’t glimmer and attract predators.

Chuck Black of the Center for Functional Nanomaterials displays a nanotextured square of silicon on top of an ordinary silicon wafer. The nanotextured surface is completely antireflective and could boost the production of solar energy from silicon solar cells.

While mimicking moth eyes to create anti-reflective surfaces is quite common (glare-free TV screens, other solar cell research) the scientists at Brookhaven used a polymer material called a “block copolymer,” which can self-organize into tiny patterns, to create a template for texturizing the surface of a silicon solar cell (solar cells make up a solar panel). Companies use self-assembling block copolymers to make things that need a textured surface, like the soles of shoes and adhesives.

A closeup shows how the nanotextured square of silicon completely blocks reflection compared with the surrounding silicon wafer.

The scientists then used the block copolymer texture as a template for creating the tiny post patterns in the silicon solar cell using a plasma of reactive gases. The chip industry commonly uses this method to manufacture electronic circuits.

Details of the nanotextured antireflective surface as revealed by a scanning electron microscope at the Center for Functional Nanomaterials. The tiny posts, each smaller than the wavelengths of light, are reminiscent of the structure of moths’ eyes, an example of an antireflective surface found in nature.

The scientists discovered that simply texturizing silicon solar cells in this way could cut down on the reflection of the cell and make it more efficient than the anti-reflective coatings commonly used today by solar cell makers. The scientists say their method outperforms a single layer of anti-reflective coatings by 20 percent. They’re still looking into how this could be a cost advantage as well.

Brookhaven National Laboratory is funded in part by the the Department of Energy Office of Science. Physicist Charles Black led the moth eye research via Brookhaven Lab’s Center for Functional Nanomaterials (CFN). The research was recently published in the journal Nature Communications and submitted for a patent. The findings could have more applications for making anti-glare windows and boosting the brightness of LEDs.

]]>Two years ago, British researcher and entrepreneur Henry Snaith, and his team at Oxford University, were experimenting with a new class of compounds and discovered something rather unusual. Using these new compounds called “perovskites,” Snaith was able to tweak a solar cell’s composition to rapidly increase the amount of sunlight that it could convert into electricity. First he reached 10 percent, then 12 percent, then 15 percent, and finally 17 percent.

He soon showed the solar cell to his co-founder, Kevin Arthur, at their four-year-old British startup called Oxford Photovolatics, which was working on another solar technology at the time. The company quickly pivoted to embrace perovskites.

The publication of Snaith’s research, which also involved scientists from Japan, in the journal Science in November 2012 helped to kickstart a love affair with perovskites in the world of solar cell material research. “A year ago there were four papers. There were 200 papers a month ago. It shows you the explosion of interest,” said Chris Case, Oxford PV’s chief technology officer, who joined Arthur and me during our interview.

Perovskites are creating an excitement that I haven’t seen in the world of solar technology research for awhile. The rapid pace at which perovskites can boost a cell’s efficiency is a major reason (check out this chart). Paltry venture capital investments in new types of solar cell materials — a result of lackluster returns from an investment boom that started around mid-2000s — has also made this intense interest around perovskites worth watching.

Oxford PV’s chief scientific officer, Henry Snaith

So far, despite the moves of Oxford Photovoltaics, the buzz is largely taking place in the academic world, though. We may never see dozens of perovskite solar startups getting billions of dollars of venture capital funding, something we saw with thin film materials CIGS (copper, indium, gallium, selenium) and cadmium-telluride.

Perovskites refer to a class of compounds with the same crystalline structure. The original perovskite was discovered in 1839 in Russia’s Ural Mountains and named after a minerologist, Lev Perovski. Most of the research since then has looked at using perovskite for electronic components, not solar cells.

That mineral named after Perovski was calcium titanium oxide. But the perovskite used by Oxford PV has no calcium, titanium or oxygen, Case said. Instead, the company uses another compound, organometallic halide, to create that same crystalline structure. Oxford PV’s perovskite contains methylammonium, lead, iodine and chlorine, and it can absorb a greater spectrum of light than silicon, the material that makes up over 80 percent of the solar cells on the market today.

A perovskite tin solar cell made by University of Oxford researchers. Courtesy of University of Oxford Press Department, Flickr Creative Commons

The startup’s researchers tune the composition and structure of their perovskite to improve its sunlight-to-electricity efficiency and to make it possible for mass production. They also are developing a printing process for depositing the perovskite, which comes in a liquid, to create solar cells. After being heated, perovskite crystallizes, its atoms then self-assemble into shapes.

“We’ve got an inexpensive material to build these cells. It’s getting more efficient almost by the day,” Arthur said. “We think it will be more efficient than silicon cells after a year or two of optimizing it.”

Oxford Photovoltaics doesn’t plan to make its own perovskite cells, however. It wants to license the recipe for making the compound to a chemical company and the printing process to glass makers or silicon solar cell makers. The company plans to make money by charging a royalty for every cell or every meter of glass made.

Perovskite films, courtesy of NREL.

Silicon solar cell makers can lay down a layer of perovskite on top of their cells to boost their efficiencies, which fall mostly in mid to high teens. SunPower’s cells can go as high as 24 percent. Adding a layer of Oxford PV’s perovskite could increase a 15 percent silicon solar cell’s efficiency by 3-5 percentage points, Case said.

That’s a bold promise because increasing a cell’s efficiency by just 1 percentage point is a big deal for silicon solar cell makers, as well as manufacturers of any other types of solar cells.

Glass manufactures could use Oxford PV’s printing process to sandwich perovskite, along with other materials for tinting the windows, in between two pieces of glass. There’s a tradeoff between efficiency and transparency, though. The more light that a window has to let inside, the less the solar material will get to absorb to produce electricity.

The challenge of embedding solar

It seemed fitting for the interview to go over engineering glass for commercial buildings, given we were sitting by a huge window on the 32nd floor of the Grand Hyatt that offered a gorgeous view of downtown San Francisco.

Embedding solar cells into buildings has always been more of a nice idea instead of an economical approach. Solar cells that won’t obstruct the view that a window offers historically have done poorly in converting much sunlight into electricity. Other types of solar cells have been too expensive to make. Plus, they won’t produce as much electricity when they line one side of a building rather than its rooftop, where they get sun for longer hours each day.

For many architects and building owners, adding solar needs to generate enough financial benefits — such as enough energy savings to lower utility bills — to justify their costs. A solar technology that could produce lots of electricity at low costs would certainly offer a more attractive option.

Smart windows from View, image courtesy of View.

Oxford PV is moving toward commercializing its perovskite technology. The 17 percent cell is the best achieved in its lab so far, but making sure the technology could produce similarly efficient cells at mass production is always a big challenge for any solar startup.

The company has built out a pilot production line to demonstrate its manufacturing process. And it’s working on showing that its pervoskite will perform as promised and last over 20 years, the expected life span of solar cells on the market today. The startup plans to start field testing its technology this year.

The company has raised £7 million in venture capital and grants. Arthur is working on raising a B round of £15 million in two tranches and hopes to close the first one before the end of the year. The company is looking at opening an office in Silicon Valley.

Oxford PV is an early player in introducing perovskite to the solar market, so how well it succeeds will be closely watched.

]]>What if all it took to charge a mobile phone was setting it out in the sun? That’s the promise of a new type of solar cell that can both absorb and emit light, opening up the possibility of using it as a display.

The Nanyang Technological University research team behind the solar cell, which recently published its results in Nature Materials, said the cells could also be used to create tinted glass windows that double as lights and displays.

“Since we are already working on the scaling up of these materials for large-scale solar cells, it is pretty straightforward to modify the procedures to fabricate light emitting devices as well,” NTU materials science and engineering assistant professor Nripan Mathews said in a release. “The fact that it can also emit light makes it useful as light decorations or displays for the facades of shopping malls and offices.”

The NTU team described how the solar cell works late last year, but didn’t realize until more recently that it can emit light. It can also be tuned to display different colors.

While solar cells generally rely on silicon, the NTU version gets its interesting properties from perovskite, a mineral with superconductive abilities. NTU reports that the cells are five times cheaper to produce than the silicon-based variety because of a simplified manufacturing process that works at room temperature.

]]>Graphene is an amazing material, but it has its flaws. Electricity can run through it at super-fast speeds, but graphene in its natural form can’t start and stop its flow, limiting its use for applications like solar cells.

One potential fix is to pair it with other materials, creating a hybrid that borrows needed characteristics from both ingredients.

A Northwestern University graduate student recently discovered a way to combine graphene with silver, which has interesting optical properties and is already widely used in solar cells. Pairing the two up until now eluded scientists because silver is not compatible with the most common technique for making graphene.

Though there are other existing and emerging approaches, graphene is generally made in an oven stocked with gases like methane and propane. At high temperatures, graphene naturally pulls out of the gases and deposits itself in sheets on copper placed in the oven. But when you replace the copper with silver, it doesn’t work.

“The methane won’t break down, it’ll just hit the hot silver and bounce off and remain methane, so there’s no carbon source to actually grow the graphene,” Argonne National Laboratory staff scientist Nathan Guisinger said in a release.

The secret was to make sure the silver was highly pure and flat. Researchers from Northwestern and Argonne covered the silver in ions that ate away impurities, then ensured its surface was totally flat. They kept the silver in a high-vacuum environment, which limited its exposure to air and water.

It worked: Graphene deposited onto the silver.

By using the new technique, the team was also able to grow graphene at around 1400 degrees Fahrenheit, down from the more usual 1800+ degrees. Working at a lower temperature slows down the growth process, which could allow for more control over the graphene.

The researchers think their work could have applications in light detectors and light absorbing devices like solar cells. They are also interested in pairing graphene with other atom-thick materials.

]]>If you look out across the SoMa district in San Francisco, you see many rooftops covered in large solar panel arrays. In the future, they might not be so obvious to the eye, as teams of researchers are working on transparent solar cells that could be worked into buildings’ windows.

The leading transparent solar cells work by absorbing as much light as possible outside of the visible range. If a cell absorbs visible light it becomes reflective, rendering it opaque instead of transparent.

At UCLA, researchers are working on cells that absorb infrared light, which can’t be seen by the unaided eye. They announced today that they have created a strip of transparent solar cells that converts 7.3 percent of the solar energy it receives into electricity. That is far below the 15 percent to 20 percent efficiency of conventional solar cells, but it is nearly twice as efficient as the 4 percent UCLA achieved last year. MIT hit 2 percent with their transparent solar cells this year.

They achieved the boost in efficiency by sandwiching two layers of solar cells together. One layer would absorb 40 percent of the infrared light that hits it, but together the two layers absorb 80 percent.

“Using two solar cells with the new interfacial materials in between produces close to two times the energy we originally observed,” team lead Yang Yang said in a release. “We anticipate this device will offer new directions for solar cells, including the creation of solar windows on homes and office buildings.”

Solar films like the UCLA team’s could also give gadgets like phones and computers a longer battery life by turning their screens into a solar panel. Wysips, for example, is testing a screen that could boost phone battery life by 20 percent.

]]>Gold may not seem like the best material to use to reduce costs, but scientists think it could help produce the most affordable solar cells yet. Stanford University researchers announced Thursday that they used minute globs of gold to create the thinnest light absorbers ever. Solar cells made with the gold nanodots would require much less material, potentially making them cheaper.

The researchers coated wafers with 520 billion gold nanodots per square inch in a honeycomb-like pattern. The dots measured 14 nanometers tall and 17 nanometers wide — thousands of times thinner than a sheet of paper, though still thicker than graphene. They coated different sets of nanodots with tin sulfide, zinc oxide and aluminum oxide to test their properties.

The wafer, dots and coating combined to create a light absorber 1,000 times thinner than commercially available options. The previous record was three times thicker.

These three gold nanodots each measure in at about 17 nanometers wide and 14 nanometers tall.

“It’s a very attractive technique, because you can coat the particles uniformly and control the thickness of the film down to the atomic level,” study lead Carl Hagglund said in a release. “That allowed us to tune the system simply by changing the thickness of the coating around the dots.”

They modified the thickness so that the wafer and nanodots would best absorb reddish-orange light. The wafers absorbed 99 percent of the light, while the dots absorbed 93 percent.

“Much like a guitar string, which has a resonance frequency that changes when you tune it, metal particles have a resonance frequency that can be fine-tuned to absorb a particular wavelength of light,” Hagglund said. “We tuned the optical properties of our system to maximize the light absorption.”

Many materials are capable of absorbing 90 percent or more of certain light wavelengths, or colors. When they are incorporated into a solar cell, the total efficiency drops. The best commercial cells out there right now hit about 15 to 20 percent efficiency. Better options could be on their way.

Ideally, a solar cell would be able to absorb all visible light, instead of just certain wavelengths. The researchers have not shown how that can be done with gold nanodots, but they plan to go ahead and demonstrate that they work in actual solar cells. They also plan to look into different nanodot materials, including silver, which is less expensive and a better absorber. They are using gold for now because it is more stable.

Graphene is made of a single layer of carbon atoms that are bonded together in a repeating pattern of hexagons. Graphene is one million times thinner than paper; so thin that it is actually considered two dimensional.

Carbon is an incredibly versatile element. Depending on how atoms are arranged, it can produce hard diamonds or soft graphite. Graphene’s flat honeycomb pattern grants it many unusual characteristics, including the status of strongest material in the world. Columbia University mechanical engineering professor James Hone once said it is “so strong it would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran Wrap,” according to the university.

These single layers of carbon atoms provide the foundation for other important materials. Graphite — or pencil lead– is formed when you stack graphene. Carbon nanotubes, which are another emerging material, are made of rolled graphene. These are used in bikes, tennis rackets and even living tissue engineering.

How was it discovered?

Chances are good that you have made graphene many times in your life. Draw a line with a pencil and small bits of graphene will flake off. But no one had both the tools and interest to reliably isolate free-standing graphene until the early 2000s.

Graphene was first studied theoretically in the 1940s. At the time, scientists thought it was physically impossible for a two dimensional material to exist, so they did not pursue isolating graphene. Decades later, interest picked up and researchers began dreaming up techniques to peel apart graphite. They tried wedging molecules between layers of graphene and scraping and rubbing graphite, but they never got to a single layer. Eventually, they were able to isolate graphene on top of other materials, but not on its own.

In 2002, University of Manchester researcher Andre Geim became interested in graphene and challenged a PhD student to polish a hunk of graphite to as few layers as possible. The student was able to reach 1,000 layers, but could not hit Geim’s goal of 10 to 100 layers. Geim tried a different approach: tape. He applied it to graphite and peeled it away to create flakes of layered graphene. More tape peels created thinner and thinner layers, until he had a piece of graphene 10 layers thick.

[youtube=http://www.youtube.com/watch?v=rphiCdR68TE&w=420&h=315]

Geim’s team worked at refining their technique and eventually produced a single layer of carbon atoms. They published their findings in “Science” in October 2004. Geim and his colleague Kostya Novoselov received the Nobel Prize in physics in 2010 for their work.

Since those first flakes made with tape, graphene production has improved at a rapid pace. In 2009, researchers were able to create a film of graphene that measured 30 inches across.

Why is it unusual?

Geim and Novoselov’s paper was wildly interesting to other scientists because of its description of graphene’s strange physical properties. Electrons move through graphene incredibly fast and begin to exhibit behaviors as if they were massless, mimicking the physics that governs particles at super small scales.

“That kind of interaction inside a solid, so far as anyone knows, is unique to graphene,” wrote Geim and another famous graphene researcher, Philip Kim, in a 2008 Scientific American article. “Thanks to this novel material from a pencil, relativistic quantum mechanics is no longer confined to cosmology or high-energy physics; it has now entered the laboratory.”

Conductive: Electrons are the particles that make up electricity. So when graphene allows electrons to move quickly, it is allowing electricity to move quickly. It is known to move electrons 200 times faster than silicon because they travel with such little interruption. It is also an excellent heat conductor. Graphene is conductive independent of temperature and works normally at room temperature.

Strong: As mentioned earlier, it would take an elephant with excellent balance to break through a sheet of graphene. It is very strong due to its unbroken pattern and the strong bonds between the carbon atoms. Even when patches of graphene are stitched together, it remains the strongest material out there.

Flexible: Those strong bonds between graphene’s carbon atoms are also very flexible. They can be twisted, pulled and curved to a certain extent without breaking, which means graphene is bendable and stretchable.

Transparent: Graphene absorbs 2.3 percent of the visible light that hits it, which means you can see through it without having to deal with any glare.

What can it be used for?

The use of graphene in everyday life is not far off, due in part to existing research into carbon nanotubes — the rolled, cylindrical version of graphene. The tubes were popularized by a 1991 paper (subscription required) and touted for their incredible physical qualities, most of which are very similar to graphene. But it is easier to produce large sheets of graphene and it can be made in a similar way to silicon. Many of the current and planned applications for carbon nanotubes are now being adapted to graphene.

Some of the biggest emerging applications are:

Solar cells: Solar cells rely on semiconductors to absorb sunlight. Semiconductors are made of an element like silicon and have two layers of electrons. At one layer, the electrons are calm and stay by the semiconductor’s side. At the other layer, the electrons can move about freely, forming a flow of electricity. Solar cells work by transferring the energy from light particles to the calm electrons, which become excited and jump to the free-flowing layer, creating more electricity. Graphene’s layers of electrons actually overlap, meaning less light energy is needed to get the electrons to jump between layers. In the future, that property could give rise to very efficient solar cells. Using graphene would also allow cells that are hundreds of thousands of times thinner and lighter than those that rely on silicon.

Intel’s transistors at 32 nanometers. More transistors helped pave the way for cheaper computing.

Transistors: Computer chips rely on billions of transistors to control the flow of electricity in their circuits. Research has mostly focused on making chips more powerful by packing in more transistors, and graphene could certainly give rise to the thinnest transistors yet. But transistors can also be made more powerful by speeding the flow of electrons — the particles that make up electricity. As science approaches the limit for how small transistors can be, graphene could push the limit back by both moving electrons faster and reducing their size to a few atoms or less.

Transparent screens: Devices such as plasma TVs and phones are commonly coated with a material called indium tin oxide. Manufacturers are actively seeking alternatives that could cut costs and provide better conductivity, flexibility and transparency. Graphene is an emerging option. It is non-reflective and appears very transparent. Its conductivity also qualifies it as a coating to create touchscreen devices. Because graphene is both strong and thin, it can bend without breaking, making it a good match for the bendable electronics that will soon hit the market.

Graphene is still in an infantile stage compared to developed materials like silicon and ITO. In order for it to be widely adopted, it will need to be produceable in large quantities at costs equal to or lower than existing materials. Emerging roll-to-roll, vapor deposit and other production techniques hint that this is possible, but they’re not yet ready to bring graphene to every mobile device screen out there. Researchers will also need to continue to work at improving graphene’s transparency and conductivity in its commercial form.

Roll-to-roll manufacturing could allow graphene to be made at large scales. Korea University

While graphene shows promise for transistors, it has a major problem: It can’t switch the flow of electricity “off” like materials such as silicon, which means the electricity will flow constantly. That means graphene can’t serve as a transistor on its own. Researchers are now exploring ways to adjust it and combine it with other materials to overcome this limitation. One technique involves placing a layer of boron nitride–another one-atom-thick material–between two layers of graphene. The resulting transistor can be switched on and off, but the electrons’ speed is slowed somewhat. Another technique involves introducing impurities into graphene.

Graphene may also be emerging too late for many of its possible applications. Electric car batteries and carbon fiber could be made with graphene, but they already rely on activated carbon and graphite, respectively — two very inexpensive materials. Graphene will remain more expensive for the time being, and may never be inexpensive enough to convince manufacturers to switch.

The world is only a decade into exploring what it can do with graphene. In contrast, silicon has been around for nearly 200 years. At the pace research is moving, we could know very soon if graphene will become ubiquitous or just another step in discovering the next wonder material.

]]>Harvard gave a big gift to the solar community today: a database of 2.3 million materials that highlights those with the most potential to be used in solar cells. All of the materials are carbon based, which researchers believe could someday replace silicon cells due to its low cost and flexibility.

The database includes more than 35,000 materials that are at least 10 percent efficient at converting solar energy into electricity. The best are 11.13 percent efficient, which is in line with the current record for organic materials. But all of them degrade to 4 to 5 percent when actually incorporated into a solar cell. Most silicon solar cells hit at least 15 percent.

Much more research will need to be done before organics are in line with silicon. Scientists will need to increase their efficiency and prevent them from degrading when they come in contact with the environment, ideally increasing their lifetime to more than 10 years. Because the database is open access, any researcher can search it to pinpoint the most ideal material for their line of research. Materials are searchable by features like their efficiency, mass and energy level, eliminating the need for labs to identify and catalogue them on their own.

Harvard researchers built the database with IBM’s World Community Grid, a program that invites volunteers to donate their unused computing time to scientific projects. The equivalent of more than 17,000 years of computing time resulted in 400 TB of data and 20,000 material characterizations a day.

The Georgia Institute of Technology and University of Wisconsin-Madison announced they will create material innovation institutes with $15 million in funding. They will launch an accelerator network with the University of Michigan. The National Institute of Standards and Technology will also fund an accelerator with $25 million.

Following Harvard’s open-access example, Lawrence Berkeley National Laboratory, MIT and a private research and development company will develop software tools that predict material behavior. Digital modeling could make it 10 times faster to develop a new material.

]]>Bacteria that live almost a mile under the surface of the ocean, where light is scare, have adapted biological ways to harness tiny amounts of light very efficiently, and in some cases can use photosynthesis to convert 100 percent of the light they find into electricity. In contrast a typical solar panel commonly converts around 15 percent of sunlight into electricity.

Now researchers at the University of Cambridge are studying the light-harvesting proteins of the deep sea Green Sulfur Bacteria to see if they can provide breakthroughs for solar energy and other electricity devices. The research is in an area called quantum biology, and the scientists say it falls outside of ‘classical’ physics, and into quantum physics.

Organisms that do photosynthesis use a network of pigments held in place by protein structures, or what scientists call pigment-protein complexes, where electrons are harvested. In many organisms as the electrons move through these systems they lose energy. But in the Green Sulfur Bacteria they are able to move electrons through their photosynthesis system to the point of harvest without losing that energy on the way.

Raising the efficiency of solar cells (which make up solar panels) is very important work for solar companies. The higher the efficiency of the cells, the more electricity can be created by the panel and the fewer cells and panels needed.

Solar companies have been working diligently on these innovations in recent years, as the basic low efficiency solar panels become more and more commoditized. For example, Alta Devices, makes cells that can convert a whopping 28.8 percent of sunlight into electricity. But those types of cells are far more expensive than the standard cells, and many are still in the research and development phase.

]]>Despite solar manufacturers’ struggles with bankruptcies and an oversupply problem this year, 2012 actually witnessed a dramatic growth in the world’s use of solar power. In particular the amount of solar panels installed on rooftops in the U.S. soared in recent quarters, helped by new financing models by companies like SolarCity and rock-bottom silicon prices — the main ingredient in solar panels.

2). The increase in solar production has to do in part with the growth of solar panel installation and capacity in the U.S. Solar panel installation is expected to rise nearly 70 percent this year, according to the Solar Energy Industry Association’s third-quarter report.

3). SolarCity’s IPO in late 2012 was one of the rare success stories for solar and cleantech startups. While some solar stocks have taken hits this year — due to over supply and super low prices — SolarCity was able to go public and its stock rose 50 percent on its first day of trading. The company, which finances and installs rooftop solar panels, originally priced its shares lower than expected, but the company’s stock price remains relatively high.

4). The price of silicon — the main material in solar panels — has dropped by half in less than two years, making solar panels cheaper to produce and to buy. The low prices, in turn, have fueled the growth of solar panel installations.

5). As is common with new commodity industries that grow rapidly, solar cells and panels are now so cheap to produce and make that there’s an oversupply problem. According to SEIA, there’s now 70 GW worth of solar module manufacturing capacity, but the current world capacity for solar modules is only 31 GW.